The present invention relates to the field of bootstrapping circuits for amplifiers, and more particularly, to preamplifier circuits used with electrical fieldmeters, especially those meters with vibrating capacitor detectors.
One use of amplifiers, especially preamplifiers, is to increase the input impedance of a circuit. Certain electrical fieldmeters that contain vibrating capacitor detectors use preamplifiers for this purpose because a high input impedance allows most of the detected signal from the vibrating capacitor detector to be received by the remainder of the detection circuitry coupled to the preamplifier. Vibrating capacitor detectors are described in U.S. Pat. No. 3,611,127 to Vosteen and a paper by Mr. Vosteen entitled "D.C. Electrostatic Voltmeters And Fieldmeters" presented at the 9th Annual Meeting of IEEE-Industry Application Society (October, 1974). Both the patent and the article are incorporated herein by reference.
In vibrating capacitor detectors, an electromechanical driver vibrates an electrode adjacent an aperture in order to measure a surrounding electric field, for example, from a charged surface under measurement located near the aperture. Both the Vosteen patent and the article show such a detector as well as the use of feedback to drive the detector shield to the same potential as the electrode. This feedback creates an essentially zero field condition which does not disturb the charge distribution on the surface under measurement.
Prior attempts to increase the input impedance of circuitry coupled to a vibrating capacitor electrode have used techniques referred to as "AC bootstrapping" techniques. FIG. 1 shows an example of one such bootstrapping technique where the vibrating capacitor detector includes a vibrating electrode 1 and a driven shield 2 to detect the electric potential on a surface under measurement 10.
Oscillator 20 drives a coil which vibrates electrode 1 at a prescribed frequency, typically 700 Hz. The voltage induced on electrode 1 is one input to a preamplifier circuit which includes an LM741-type operational amplifier 30 (i.e., a high impedance type) configured as a voltage follower. The output of amplifier 30 is protected by zener diodes 33 and 34 which are connected in series and with opposing polarities between the output of operational amplifier 30 and ground. The values of the zener diodes depend on the supply voltage. For example, 11 volt zeners would be used for a 15 volt supply voltage.
Resistances R1-R3 and capacitor C1 provide AC bootstrapping. R2 and R3 are connected in series between ground and the noninverting input of operational amplifier 30. That input is also connected to vibrating electrode 1. R1 and C1 are connected in series between the output of amplifier 30 and the junction of resistors R2 and R3. The values of resistors R1-R3 and C1 are chosen to maximize the input impedance of the preamplifier at the frequency of interest, i.e. oscillator 20's frequency.
This type of A.C. bootstrapping was designed to maximize the transmission of signals at the desired (i.e., oscillator) frequency and to minimize the interference from signals at other frequencies. AC bootstrapping was also used to circumvent the disadvantages of voltage offset that occurred when the preamplifier's input impedance was increased by increasing D.C. resistance. For example, leakage currents inherent in operational amplifiers will flow through a resistance at the input of an operational amplifier and create undesirable offset voltages; the higher the resistance, the higher the offset.
AC bootstrapping, however, has several problems, including a lack of stability. The circuit in FIG. 1 is highly tuned. Stray signals may cause it to resonate at the tuned frequency even when there is no voltage change on the surface under test. The result is improper detection of fields and false alarms if the system is used as an alarm circuit. Furthermore, resistances R2 and R3 still cause undesirable voltage offsets and, at the tuned frequency, both the noise and the signal are amplified making it difficult to improve signal/noise ratios.
One object of the present invention is to increase an amplifier's input impedance without increasing its offset voltage.
Another object of the invention is an amplifier with a better signal/noise ratio than one employing AC bootstrapping.
A further object of the invention is an electric fieldmeter with high accuracy and stability.